netfilter: nf_tables: Make nft_meta expression more robust

nft_meta_get_eval()'s tendency to bail out setting NFT_BREAK verdict in
situations where required data is missing leads to unexpected behaviour
with inverted checks like so:

| meta iifname != eth0 accept

This rule will never match if there is no input interface (or it is not
known) which is not intuitive and, what's worse, breaks consistency of
iptables-nft with iptables-legacy.

Fix this by falling back to placing a value in dreg which never matches
(avoiding accidental matches), i.e. zero for interface index and an
empty string for interface name.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 1 insertions, 5 deletions

diff --git a/net/bridge/netfilter/nft_meta_bridge.c b/net/bridge/netfilter/nft_meta_bridge.c
index bed66f536b34..a98dec2cf0cf 100644
--- a/net/bridge/netfilter/nft_meta_bridge.c
+++ b/net/bridge/netfilter/nft_meta_bridge.c
@@ -30,13 +30,9 @@ static void nft_meta_bridge_get_eval(const struct nft_expr *expr,
         switch (priv->key) {
         case NFT_META_BRI_IIFNAME:
                 br_dev = nft_meta_get_bridge(in);
-                if (!br_dev)
-                        goto err;
                 break;
         case NFT_META_BRI_OIFNAME:
                 br_dev = nft_meta_get_bridge(out);
-                if (!br_dev)
-                        goto err;
                 break;
         case NFT_META_BRI_IIFPVID: {
                 u16 p_pvid;
@@ -64,7 +60,7 @@ static void nft_meta_bridge_get_eval(const struct nft_expr *expr,
                 goto out;
         }
 
-        strncpy((char *)dest, br_dev->name, IFNAMSIZ);
+        strncpy((char *)dest, br_dev ? br_dev->name : "", IFNAMSIZ);
         return;
 out:
         return nft_meta_get_eval(expr, regs, pkt);